In recent years, researches are carried out widely on the technology in which an amorphous semiconductor film or a crystalline semiconductor film (semiconductor film having crystallinity of not single crystal but polycrystal, microcrystal or the like) formed over an insulating substrate of glass or the like, that is, a non single crystal silicon film, is crystallized or the crystallinity is promoted by performing laser annealing. A silicon film is frequently used for the semiconductor film.
A glass substrate is inexpensive and rich in workability compared with a quartz substrate that has been conventionally used frequently, and is provided with an advantage of capable of easily fabricating a substrate having a large area. This is the reason for carrying out the above-described researches. Further, laser is preferably used for crystallization because in the laser process, a substrate is not heated and the process is suitable for using a glass substrate having low heat resistance.
A crystalline silicon film formed by performing laser annealing is provided with high mobility. When such a crystalline silicon film is used, TFTs (Thin Film Transistor) for driving pixel and for drive circuit can be integrated on one sheet of glass substrate.
Generally, the laser spot of laser beam is provided with dimensions of several centimeters or less, respectively, and therefore, a special device is needed in carrying out processing in respect of a large area.
Generally, pulse laser beam of excimer laser or the like is fabricated by an optical system such that a square spot of several centimeters square is formed and the laser beam is scanned (irradiated position of laser beam is moved relatively with respect to the irradiated face) thereby performing laser annealing.
Further, there is known a technology where laser beam is fabricated into a linear shape (several millimeters width).times.(several tens centimeters) and irradiated while being scanned in a direction of a width of the linear beam.
When the method is used, different from the case where laser beam having a spot-like shape in which scanning in the forward and rearward direction and the left and right direction is needed, is used, the laser irradiation can be carried out on an entire irradiated face by scanning the linear laser only in a direction orthogonal to the linear direction and high productivity can be achieved.
Although the method of performing laser annealing in respect of a non single crystal semiconductor film by scanning the pulse laser beam fabricated in the above-described linear shape, is a method excellent in the productivity, several problems have been posed.
One of particularly serious problems among them is that laser annealing cannot be performed uniformly over the entire film surface.
When the linear laser began to be used, a phenomenon where stripes were formed at portions of overlapping linear beams was caused significantly and characteristics of a semiconductor considerably differed among the stripes.
FIG. 1a shows an optical photograph of a crystalline silicon film provided when the laser beam (KrF excimer laser) having a linear shape extending in the transverse direction of paper face is irradiated by scanning the beam from bottom toward top of paper face.
As apparent from FIG. 1a, stripe patterns are observed at portions where linear beams overlap. The stripe patterns reflect a difference in crystallinity in the film.
When a liquid crystal display is fabricated by using, for example, the film shown by FIG. 1a, there causes inconvenience where the stripes are shown on the screen as they are.
The reason is that a variation in the characteristic of the fabricated TFT emerges by reflecting a difference in crystallinity in the film having stripes as shown by FIG. 1a.
It can be reduced to a nonproblematic level by devises as follows.
(1) Improvement of an non single crystal semiconductor film that is an object of irradiating laser is improved. PA1 (2) Making a scan pitch (interval between contiguous linear laser beams) of a linear laser fine. PA1 (3) Pursuit of an optimum combination of parameters determining various irradiating conditions. For example, a combination of parameters of scan pitch of linear laser, scan speed, pulse oscillation interval, irradiation energy density and the like is optimized. PA1 (1) The original energy distribution of the rectangular beam is inherently provided with an energy distribution having a striped shape. PA1 (2) The cause is derived from lens groups utilized in forming the linear laser beam. PA1 means for irradiating a linear laser beam, PA1 means for irradiating the laser beam by scanning the laser beam in a direction having a predetermined angle .theta. (.theta..noteq.0.degree.) in respect of a direction of a width of the linear laser beam, PA1 wherein the laser beam is provided with an intensity distribution periodically varied in a longitudinal direction of the laser beam; PA1 wherein the laser beam is of a pulse oscillation type, and PA1 wherein pulses of the laser beam are irradiated by overlapping portions of the pulses at an irradiated region. PA1 means for irradiating a linear laser beam, PA1 means for irradiating the laser beam by scanning the laser beam in a direction having a predetermined angle .theta. (.theta..noteq.0.degree.) in respect of a direction of a width of the linear laser beam, PA1 wherein the laser beam is provided with an intensity distribution periodically varied in a longitudinal direction of the laser beam, PA1 wherein the laser beam is of a pulse oscillation type, PA1 wherein pulses of the laser beam are irradiated by overlapping portions of the pulses at an irradiated region, and PA1 wherein the angel .theta. is selected from a range where peaks of the periodic intensity distribution do not overlap.
When the above-described stripe patterns were made inconspicuous, the nonuniformity of energy distribution of the laser beam per se began conspicuous.
Generally, when the linear laser beam is formed, a beam originally having a rectangular shape is fabricated into that of the linear shape by passing the beam through pertinent lens groups.
The aspect ratio of the beam having a rectangular shape falls in a range of about 2 through 5 and the beam is deformed into a linear beam having an aspect ratio of 100 or higher by lens groups (referred to as beam homogenizer).
In this case, the lens groups are designed to homogenize simultaneously the energy distribution in the beam. According to the method of making uniform the energy distribution, the original rectangular beam is divided and thereafter, the divided portions of the beam are respectively magnified and overlapped to homogenize the beam.
In respect of the beam which has been divided and reconstructed by such a method, apparently, the finer the division the more uniform the energy seems to be distributed.
However, when the beam is actually irradiated on a semiconductor film, despite the fineness of the division, stripe patterns as observed in FIG. 1b are formed on the film.
The stripe patterns emerge to extend in a direction of the width of the linear beam. That is, FIG. 1b shows a silicon film that is produced by irradiating a laser beam having a longitudinal direction in the left and right direction of paper face by scanning the laser beam from bottom toward top of paper face. The stripe patterns shown by FIG. 1b orthogonal to the stripe patterns shown by FIG. 1a caused by way of overlapping linear laser beams, are shown on the film.
Incidentally, although stripes in the vertical direction are observed also in FIG. 1a, in this case, the photographing conditions are set such that horizontal stripes are easy to observe and therefore, the vertical stripes do not emerge so significantly as shown by FIG. 1b.
Further, when stripes in the vertical direction slightly observed in FIG. 1a are made easy to observe, vertical stripes as shown by FIG. 1b are observed. That is, nonuniformity of annealing (nonuniformity of crystallinity) represented by the vertical stripes in FIG. 1a and FIG. 1b are actually in the same state.
The stripe patterns in the vertical direction of FIG. 1a and FIG. 1b are formed in an innumerable number orthogonally to the longitudinal direction of the linear laser beam.